1. Field of the Invention
The present invention relates to a heat treating process for achieving a forced ageing of a commutator for a rotary electrical machine, by supplying heat during rotation.
2. The Prior Art
The commutator in question consists of a plurality of segments arranged tangentially one after the other and with intermediate insulating plates. These segments are clamped in the axial direction so that together they form a rigid body. During operation of the electric motor, the commutator is heated and its segments tend to expand, which, because of the clamping, results in the segments being bent to a great or a small extent radially outwards, radially inwards or in a tangential direction. Since some of the deformations connected herewith, for example deformations in the insulating material, are of a permanent nature, the different segments of a newly-made machine will at first change their shape and/or positions at each new heating. Consequently it is no use finish-turning the commutator as long as the stabilization of the hot commutator conditions has still not taken place.
The expression "forced ageing" refers to a heat treatment of the commutator which is carried out for the purpose of stabilization before the machine is delivered, and before the final turning of the commutator has taken place.
Attempts to achieve the above-mentioned stabilization by repeatedly heating the commutator to maximum temperature by blowing in hot air or heating in a furnace will seldom result in acceptable results, since the temperature distribution in copper, the relation between the iron and the copper temperatures and certain other circumstances prevailing during such heating differ very much from heating under normal operating conditions. The commutator looses at full load, that is the effect which to a great extent is transferred to heat in the commutator, consist of the effect caused by the voltage drop of the brushes together with the effect which is developed by the friction of the brushes against the commutator surface.
Nowadays it is commonly recognized that, in the case of forced ageing of a commutator, the heat should be supplied in a manner that corresponds as much as possible to heating in normal operation. During normal operation the commutator losses are constituted to a great extent (20 - 60%) by the friction losses and for the rest by the heat developed in connection with the current transfer between the brushes and the commutator surface. The resistance losses in the interior of the segments are insignificant. According to the ageing method which is nowadays considered to provide the best result -- the socalled block treatment -- the commutator is heated by friction heat only. Even if this simulation of the actual conditions is not quite perfect, it is all the same the question of heat which is only supplied to the radially outer surface of the segments and also over the same axial distance as in normal operation.
In the block treatment method there are arranged, for example, wooden blocks which exert a pressure on the commutator surface whereas the rotor is brought into rotation by means of an auxiliary motor. A disadvantage of the block treatment is that the heating must be allowed to take place very slowly in order that the method can be performed without the risk of a radial deformation of the segments. If one of the segments protrudes to a relatively great extent already at the start of the block treatment, this will involve a risk that a relatively great part of the released braking effect will be concentrated in the protruding segment, thus causing a further deflection.